As a device crucial to recent medical diagnostics, a magnetic resonance imaging (MRI) diagnostic system is available. This system is a method for imaging internal information within a living body by utilizing a nuclear magnetic resonance phenomenon, and requires a strong magnet. Recently, for the purpose of improvement of resolution power and processing speed, an MRI diagnostic system using an extremely strong magnet, such as 1.5 T (tesla) or 3 T, is being developed. For such strong a magnet, a superconductive magnet is generally used. It is necessary to cool the superconductive magnet down close to absolute 0 of temperature and it take a lot of time and cost to start-up. Therefore, the superconductive magnet that has started once shall be operated on a steady basis, and it shall not be stopped as long as there is no special reason.
The stronger the magnet, the stronger the attracting force of the magnet for a magnetic body. In the case of bringing tools, such as a driver made of a magnetic body, such as iron, medical supplies, such as an oxygen bottle or an infusion stand, into the MRI examination room, a magnet attraction accident where these are attracted to a magnet and absorbed into the system may occur. The magnetic body, such as an oxygen bottle, attracted by strong force cannot be detached with a human's power. In order to detach these, the superconductive magnet has to be stopped; however, it takes time to stop and restart the superconductive magnet and the diagnostic task is stopped in the meantime. Other than the time, it also requires a lot of cost.
In the case of bringing tools or medical supplies made of magnetic body by accident, the magnetic body collides into a patient or a laboratory technician and it may cause injury. Therefore, the magnetic body must not be brought into the examination room. Even in the case of transferring a patient to the examination room, it is necessary to use a stretcher or wheelchair exclusively for MRI which has smaller mass ratio of magnetic body. For other infusion stands or medical carts, medical supplies exclusively for MRI should be used.
Laboratory technicians and/or nurses who are engaged in the MRI diagnostic system are educated about and familiar to the risk of magnet, and they will basically never bring a magnetic body. Since the laboratory technicians will confirm whether or not he/she has a magnetic body, such as pacemaker, in the body in advance, there should be no problem for a patient. However, it is not guaranteed that the laboratory technicians have perfect knowledge. For example, since ferrite is not metal, it does not look like being attracted to the magnet; however, this is a magnetic body. There are some cases where a thing, which is believed not being such magnetic body, is brought in. A patient falls into an urgent condition during the examination, and there is a case that a nurse brings an oxygen bottle in by accident. There are some nurses who do not know about the MRI diagnostic system in detail. There are other cases where some workers who have no knowledge about the risk of magnet bringing a stepladder, tools and/or cleaning implements for the purpose of maintenance or cleaning of the MRI examination room.
In order to prevent any magnet attraction accident due to bringing a magnetic body in, it is necessary to mount a gate-type magnetic body detector for detecting a magnetic body and providing a warning in front of a shield door at the MRI examination room.
Furthermore, as a device similar to the gate-type magnetic body detector, a gate-type metal detector is also used at airports. This device is to detect an eddy current flowing in metal, and nonmagnetic metal, such as aluminum, is also detected. Consequently, supplies exclusively for MRI where the majority is made of non-magnetic body are also warned. Therefore, even if a metal detector is used in a shield door at an examination room of the MRI diagnostic system, it is meaningless. Inversely, the magnetic body detector does not react to non-magnetic metal. Therefore, it is meaningless to use this magnetic body detector at airports. The present invention only relates to a magnetic body detector.
As the prior art regarding such magnetic body detector, in Japanese Patent Application Laid-Open No. H5-52962 (Patent Literature 1), an invention where homogeneous magnetic fields are alternately generated using a two-way Helmholtz coil, and a change of magnetic flux density B to be generated due to invasion of a magnetic body is detected by the two-way detection coil, and an electromotive force is amplified and detection is conducted is proposed.
In addition, as the prior art regarding such magnetic body detector, in International Publication Number WO03/077725 A2 (Patent Literature 2), an invention where hall elements are arranged at both sides of a gate, and an output signal is determined by CPU is proposed.
[Patent Literature 1]                Japanese Patent Application Laid-Open No. H5-52962        
[Patent Literature 2]                International Publication WO03/077725 A2        